1. Field of the Invention
The invention relates to the manufacture of highly dense integrated circuits and more particularly to the formation of a silicon oxide isolation region within the integrated circuit.
2. Description of the Prior Art
In the manufacture of highly dense integrated circuits, individual device structures are typically separated and electrically isolated by means of a silicon oxide isolation region, also referred to as a field oxide isolation region. The isolation region is typically produced by the exposure of a silicon wafer to an oxidizing atmosphere while using an oxidation mask to protect regions which are not desired to be oxidized. These latter regions will be the location for the active device structures. One widely used technique for creating isolation regions is called LOCOS--for LOCal Oxidation of Silicon.
In the LOCOS technique, a pad oxide is grown on the surface of a silicon substrate. A silicon nitride layer is deposited on the surface of the pad oxide, and then patterned to create an oxidation mask. The exposed areas of the substrate are then oxidized to form the silicon oxide isolation regions. The oxidation mask is removed and the device structures are created in subsequent processing steps.
The LOCOS method has problems, however, especially as device geometries continue to get smaller. During oxidation, oxygen diffuses through the pad oxide to the substrate and forms an undesired "bird's beak". The bird's beak extends the silicon isolation region into the active device region, reducing the area in which devices can be built.
To reduce the bird's beak problem, a layer of undoped polysilicon is added between the pad oxide and silicon nitride layers, in what is referred to as PBLOCOS, or Polysilicon Buffered LOCal Oxidation of Silicon. An example is shown in U.S. Pat. No. 4,829,019. The polysilicon layer in this invention is intended to block oxygen from diffusing into the pad oxide and substrate, thereby reducing the bird's beak length.
Three notable problems exist, however, in the use of PBLOCOS. These are "pits" in the polysilicon layer, bird's beak, and the oxide thinning effect. FIG. 1 shows the prior art. Shown are substrate 10, pad oxide layer 12, undoped polysilicon layer 14, and silicon nitride layer 16. During formation of the field oxide 18, as shown in FIG. 2, the polysilicon in the vicinity of the field oxide tends to be weakened due to stress. Also, small regions of the polysilicon at the boundary of the field oxidation region are converted into silicon nitride during field oxidation. During etch of the silicon nitride layer, these small regions are etched out and pits 20 are formed. During the subsequent reactive ion etch of the polysilicon, these pits can extend through the pad oxide layer 12 to the silicon substrate 10 and cause damage.
Workers in the field are well aware of the pitting problem. One solution is a two-step oxidation of the polysilicon layer, after removal of the silicon nitride. The polysilicon is oxidized, then etched with a dip-back etch, then oxidized again and etched again. This eliminates the pits problem, but adds several processing steps and may cause unwanted substrate oxidation.
Another solution to the "pits" problem has been patented by L. B. Fritzinger et al in U.S. Pat. No. 5,002,898. The novel solution consists of adding a protective oxide layer between the polysilicon buffer and silicon nitride layers. Removal of the silicon nitride by wet etching does not affect the polysilicon layer due to the protection afforded by the protective oxide. The protective oxide and polysilicon layers may then be removed without damage to the substrate.
The second problem with the PBLOCOS technique is the bird's beak effect. Though the polysilicon buffer layer reduces the bird's beak length when compared with using LOCOS, it is still a problem, particularly as device geometries continue to decrease. Polysilicon oxidizes near the polysilicon/field oxide interface, and oxygen may diffuse through to the pad oxide and silicon substrate, causing the bird's beak. If doped polysilicon is used, an even longer bird's beak length results due to the faster oxidation of doped polysilicon versus undoped polysilicon.
The third problem is the "oxide thinning effect". This is a thinning of the oxide at the interface of the gate oxide and field oxide regions. The oxidized polysilicon in the PBLOCOS technique must be removed before gate oxide growth. It is desirable to have a planar topography, i.e., limit the field oxide thickness above the silicon surface, therefore as shown in FIG. 2A the dipback thickness 22 must be large. This exposes region 24, which has a slower oxidation rate than the horizontal portion of the substrate. Thus the oxide thickness in region 24 is thinner and more likely to break down.
JA 57-39551 (Miyazawa) shows a variation on PBLOCOS in which the polysilicon layer is formed on the nitride layer, which is deposited on the oxide. A photoresist is formed with an opening smaller than the poly/nitride opening to create a mask for field implant, to form an offset between the field implant and field oxidation window. The photoresist is stripped and field oxidation is completed, however the top layer of polysilicon also oxidizes since it is exposed.